Reactive instabilities in linear acidizing on carbonates

Acid injection, reactive instabilities and wormholing in carbonate reservoirs is investigated through the
analysis of the linear acidizing problem theoretically and experimentally. Theoretically acidizing was
analyzed by formulating the problem as a reactive moving boundary Stefan type problem. Wormholing
is viewed as a reactive infiltration instability to the trivial solution of uniform dissolution. A linear
stability analysis from the equilibrium state is performed and the critical wavenumber below which
instabilities have a positive growth rate is identified. When applied to the scale of the experiment,
an optimum injection velocity is identified for a given formation and injection concentration, for
the growth for a single wormhole. This optimum injection velocity scales with the inverse of the
specimen diameter. Experimentally, linear acidizing tests were performed in Mons chalk, a high
porosity analogue of North Sea reservoir chalk. In the experiments the critical injection velocity for
wormhole formation at minimum acid injection was obtained and the results were compared with
the theoretical predictions.